In a prior-art communications system, a frequency division duplex (FDD) or time division duplex (TDD) manner is generally used for communication. In an FDD system, different frequencies are used for uplink and downlink communication. In a mobile communications system in a TDD mode, receiving and sending are performed in different timeslots of a same frequency channel (that is, a carrier), and timeslots are used to separate a receive channel from a transmit channel. In both of the communication manners, signals can communicate only within a time or on a specific frequency band.
A full-duplex wireless communications technology is a technology that differs from TDD and FDD. Communication can be implemented at a same time at a same frequency by using the technology. FIG. 1 is a schematic diagram of a prior-art full-duplex communications system. As shown in FIG. 1, when two communications devices communicate at a same time at a same frequency, a receive antenna not only receives a wanted signal (indicated by a dotted line) from a peer end, but also receives a signal sent by a communications device in which the receive antenna is located, that is, a self-interference signal (indicated by a solid line). Moreover, because a transmit antenna is quite close to the receive antenna, the strength of the self-interference signal is often much higher than that of the wanted signal of the peer end. A full-duplex technology is a self-interference signal eliminating technology developed for the foregoing problem. A basic principle is that a communications device “knows” a signal transmitted by the communications device, and therefore, such a self-interference signal can be eliminated at a receive antenna by using some means.
Currently, methods for resolving a self-interference problem are basically classified into three types: antenna elimination, radio frequency elimination, and baseband digital elimination. In the radio frequency elimination, due to hardware, self-interference processing has certain frequency selectiveness: some frequency bands have good eliminating performance, but other frequency bands have relatively poorer eliminating performance. If a system is a wideband system, after the radio frequency elimination, levels of remaining noise at different frequencies in entire bandwidth greatly vary. If a difference of elimination in the entire bandwidth is greater than a signal-to-noise ratio of a transmit link, even after digital elimination is performed, remaining noise on some frequency bands still cannot be as low as a noise floor, which affects an application scope of the full-duplex technology.